Book set aside (7)
I believe the answer is:
reserve
I believe this clue is a double definition.
'book' is the first definition.
(as in booking or reserving in advance)
'set aside' is the second definition.
(I know that set aside can be written as reserve)
(Other definitions for reserve that I've seen before include "Bank" , "Kept back stock" , "Lack of openness" , "not in the front line" , "put to one side" .)